A display panel device equipped with a touch panel for touch input is disclosed in a large number of documents. For example, JP 2002-040243A (Patent Document 1), JP 2002-055780A (Patent Document 2) and JP 2002-156920A (Patent Document 3) disclose a display panel device in which a touch panel for touch input is disposed on an upper side of a display panel board. All of the touch panels described in the Patent Documents 1 to 3 are a resistive type in which two transparent electrodes are disposed in a spaced-apart and opposed relation, wherein one of the transparent electrodes disposed on a viewing side of the display panel device is adapted to be pressed and brought into contact with the other transparent electrode, thereby allowing a tough input to be sensed. In the Patent Documents 1 to 3, there is an air gap between the two electrodes, which leads to a problem that reflected light in surfaces of the electrodes is transmitted to the viewing side of the display panel device, causing deterioration in display quality. In order to cope with this problem of internal reflection, the Patent Documents 1 to 3 teach arranging a circular polarization element on the viewing side with respect to the touch panel. Specifically, the Patent Documents 1 to 3 propose to arrange a linear polarization layer and a ¼ λ retardation (phase difference) layer in this order from the viewing side of the display panel, and arrange the touch panel on a lower side of the ¼ λ retardation layer. The Patent Document 1 discloses a configuration in which a touch panel is combined with a reflective liquid crystal display, and the Patent Document 2 mentions applicability to both a liquid crystal display and an organic EL display. Further, the Patent Document 3 offers an example of an organic EL display.
JP 2010-198103A (Patent Document 4) discloses a capacitive touch input device. A touch sensor described in the Patent Document 4 comprises an electrode section provided on each of opposite sides of a substrate film formed as an integral film and composed of a patterned electrically conductive layer. The Patent Document 4 mentions that the integral film includes a configuration which comprises a functional film inseparably formed on one or each of opposite surfaces of a film body, for example, by sputtering. In the Patent Document 4, an index matching (refractive index adjusting) film is described as an example of the functional film, and FIG. 4(a) illustrates a film structure in which a plurality of high refractive index films and a plurality of low refractive index films are alternately arranged on a surface of a film body in a thickness direction of the film body. The Patent Document 4 further discloses, in FIG. 4(c) thereof, a film structure in which a single layer of low refractive index film is formed on a surface of a substrate film. The index matching film and the low refractive index film described in the Patent Document 4 is explained that they have a function of preventing a reflectance from largely changing between a region with the electrode section and a region devoid of the electrode section. The touch panel sensor comprising the integral film and the electrode section on each of the opposite sides of the integral film is used in such a manner that a protective cover functioning as an input screen is bonded to one surface of the touch panel sensor through an adhesive layer, and a display formed as a flat panel display is bonded to the other surface through an adhesive layer.
A touch sensor having a configuration in which an electrode is formed on each of opposite sides of a dielectric film layer is also described in FIG. 5 of JP 2009-076432A (Patent Document 5). The Patent Document 5 further describes, in FIG. 7 thereof, a double-electrode type touch sensor prepared by: forming a transparent electrode layer on one surface of a film substrate through an undercoat layer; bonding the other surface of the film substrate to a second film substrate through an adhesive layer; and forming a second transparent electrode layer on the second film substrate through a second undercoat layer. The Patent Document 5 mentions that a difference between respective refractive indexes of the undercoat layer and the transparent electrode layer is preferably set to 0.1 or more. Although the Patent Document 5 mentions that the undercoat layer may be formed in a multi-layer structure, a relationship therebetween in terms of thickness is not particularly described. Moreover, in the case where the undercoat layer is formed in a two-layer structure, a relationship between the undercoat layers in terms of refractive index is not particularly described. JP 2008-140130A (Patent Document 6) describes a touch sensor in which an electrode section composed of a patterned electrically conductive film is formed on each of opposite surfaces of a transparent substrate made, for example, of PET, wherein a protective layer is formed on an outer side of one of the two electrode sections, and a sensitivity adjusting layer is formed on an outer side of the other electrode section. In this structure, the sensitivity adjusting layer is designed such that by a thickness thereof is adjusted to a given value to prevent potential fluctuation at a grounding electrically conductive pattern of the touch sensor.
With regard to patterning of a transparent electrode for use in a touch sensor, one example is illustrated in FIG. 3 of the Patent Document 4, and some other examples are described in detail in WO 2006/126604A (Patent Document 7).
JP 03-009323A (Patent Document 8) teaches selecting an appropriate material under recognition of a necessity to achieve optical isotropy without the occurrence of colored interference fringes, when a resin material is used in an electrode substrate for a liquid crystal display panel. JP 3542838B (Patent Document 9) and JP 3569557B (Patent Document 10) describe using an optically isotropic resin material as a material for an electrically conductive transparent sheet for a touch panel. In the Patent Documents 8 and 9, a recommended optically isotropic resin material includes a polycarbonate-based resin, a polyethersulfone-based resin, a polysulfone-based resin, and a polyarylate-based resin.